74 HEAT AND ENERGY 



Experiment 66. Fill two thin bottles with water and cork 

 them tightly. Heat one and allow the other to freeze. The water 

 may be left to freeze over night ; but do not try to heat the 

 tightly closed bottle without help from your instructor. Note 

 and draw conclusions from the results. 



Careful study has shown that water has its smallest 

 volume at 4- Centigrade. If heated above or cooled 

 below that point, it expands. 



84. Changes of State. Early in our study we learned, 

 that solids change to liquids and liquids to gases upon 

 being heated ; also that gases become liquids and liquids 

 change to solids when cooled ( 9). In some substances 

 these changes occur at ordinary temperatures, and are 

 common enough. In other substances, however, the 

 changes would need such a high or low degree of heat 

 that they are seldom or never accomplished. 



The change from a solid to a liquid is called melting, 

 fusion, or liquefying; the change from a liquid to a solid 

 is called solidifying. The temperature at which a solid 

 substance liquefies is the same as that at which it solidi- 

 fies from a liquid state. Vaporization is the change from 

 a liquid to a gaseous state; the temperature at which a 

 substance vaporizes is called its boiling point. 



In a large number of substances pressure upon them 

 raises the temperature at which these changes of state 

 occur. Thus water, which can usually be no hotter than 

 100 C., rises to a much higher degree in a locomotive 

 boiler, where it is under pressure from the steam. With 

 substances such as ice, which contract when they are 

 melting, pressure lowers the melting point. Thus a 

 block of ice receives the imprint of a dish which rests 



